Process for cleaning a semiconductor wafer

ABSTRACT

Semiconductor wafers are cleaned by forming a first liquid film on a surface of the semiconductor wafer to be cleaned, the first liquid film containing hydrogen fluoride and ozone; replacing the first liquid film with a second aqueous liquid film which contains hydrogen fluoride and ozone, the concentration of hydrogen fluoride in the second liquid film being lower than in the first liquid film; and removing the second liquid film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for cleaning a semiconductor wafer with an aqueous liquid film, which contains hydrogen fluoride and ozone.

2. Background Art

Semiconductor wafers for use in the fabrication of electronic components must generally be cleaned of particles and metal contaminants. Such cleaning steps are customary both for the fabricators of the electronic components and for their suppliers, i.e. the manufacturers of the semiconductor wafers.

Aqueous solutions which contain hydrogen fluoride (HF) and ozone (O₃) have proven to be effective cleaning agents. U.S. Pat. No. 5,759,971 describes a cleaning process in which a plurality of semiconductor wafers are simultaneously immersed in an aqueous bath which contains HF in a concentration of between 0.03 and 0.05 wt % and ozone dissolved to saturation. Modem processes are designed for single-wafer treatment, inter alia because this allows lower consumption of cleaning liquid. The cleaning liquid in such a process is applied as a liquid film onto one or both major surfaces of the semiconductor wafer. U.S. Pat. No. 7,037,842 B2 describes a process in which a surface of a rotating semiconductor wafer is sprayed with an aqueous cleaning liquid which contains hydrogen fluoride and ozone.

For economic reasons it is desirable to make the cleaning cycle times as short as possible, i.e. to achieve the highest possible throughput of semiconductor wafers. Conflicting with this aim is the fact that, in general, cleaning is commensurately more complete when it is carried out for a longer time. An obvious solution to this problem might be supposed to involve increasing the concentration of the hydrogen fluoride and ozone components which exert the cleaning action. This strategy, however, has the disadvantage that increasing the concentration of hydrogen fluoride will also enhance the etching action of this component, which leads to undesirable roughening of the cleaned surface.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a process which allows short cycle times and effective cleaning without unacceptably increasing the roughness of the surface. These and other objects are achieved by a process for cleaning a semiconductor wafer, comprising forming a first aqueous liquid film on a surface to be cleaned, the first liquid film containing hydrogen fluoride and ozone; replacing the first liquid film with a second aqueous liquid film which contains hydrogen fluoride and ozone, the concentration of hydrogen fluoride in the second liquid film being lower than in the first liquid film; and removing the second liquid film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The advantages of the process are significant, particularly in connection with the cleaning of silicon semiconductor wafers which are still free of semiconductor components. Such semiconductor wafers are typically cleaned after polishing, heat treatment or after the deposition of an epitaxial layer. The etching action of the cleaning liquid exposes COP defects (crystal originated particles) and oxygen precipitates (BMD, bulk microdefects). These are detected as particles by scattered-light measuring instruments, and they are a cause of the increase in the roughness of the surface. The invention makes it possible to restrict this detrimental effect, but without having to abandon rapid completion of the cleaning.

The cleaning process according to the invention, which is based on the concept of single-wafer treatment of a rotating semiconductor wafer, comprises two cleaning stages which differ primarily by the concentration of hydrogen fluoride in the cleaning liquid, the concentration in the cleaning liquid of the first stage being higher than in the cleaning liquid of the subsequent stage. The first stage extends from the formation of a first liquid film on the semiconductor wafer's surface to be cleaned, until replacement of the first liquid film with a second liquid film. The second stage lasts from this time until the second liquid film is removed from the semiconductor wafer's surface to be cleaned. Both stages preferably last no longer than 60 s respectively, more preferably no longer than 30 s respectively, so that cleaning according to the invention can be carried out in preferably no more than 120 s, more preferably in no more than 60 s.

The changeover from the first cleaning stage to the second cleaning stage is preferably carried out through the first liquid film being displaced by the second liquid film, and the semiconductor wafer's surface to be cleaned remains constantly wetted with liquid during this time.

The thickness of the first and second liquid films is controlled via the speed with which the semiconductor wafer is rotated about a rotation axis which perpendicularly intersects the center of the surface to be cleaned. The rotation speed preferably lies in a range of from 100 to 2000 rpm. The range of from 200 to 500 rpm is particularly preferred. The speed of the rotational movement during the first cleaning stage may differ from the speed of the rotational movement during the second cleaning stage. The speed selected for the first cleaning stage will preferably not be changed during the cleaning stage. Owing to the rotational movement of the semiconductor wafer, used cleaning liquid flows off together with particles and dissolved contaminants at the edge of the semiconductor wafer. The loss of cleaning liquid due to this flow is compensated continuously by applying a corresponding amount of fresh liquid through one or more nozzles onto the semiconductor wafer's surface to be cleaned.

The first cleaning stage is essentially intended to rapidly dissolve native surface oxide and possibly polishing agent residues containing silica sol, with the aid of a comparatively high concentration of hydrogen fluoride. The adhesive base for insoluble particles is also removed by this, so that they can continue to be washed off from the surface of the semiconductor wafer. The second stage is in essentially intended to sustain the washing process under conditions which promote it, while simultaneously minimizing the material erosion generated by the etching of semiconductor material.

In the first liquid film, the concentration of hydrogen fluoride is preferably from 0.1 to 10 wt %, and it more preferably lies in a range of from 0.1 to 2.0 wt %. In the second liquid film, the concentration of hydrogen fluoride is lower, preferably from 0.001 to 0.1 wt %, and it more preferably lies in a range of from 0.02 to 0.05 wt %. The concentration of ozone may be the same in both liquid films, or it may be lower in the second liquid film than in the first liquid film. In any event, however, it should be selected so that it is high enough to leave behind a hydrophilic wafer surface. Ozone may already be contained in the cleaning liquid when the liquid film is applied onto the semiconductor wafer. A preferred process, however, is one in which the liquid film is enriched with ozone by diffusion-driven transport from the surrounding gas phase. Such a process is described, for example, in U.S. 2002/0050279 A1. In this case, ozone is introduced into the process chamber as a mixture with oxygen. The concentration of ozone in oxygen is preferably from 3 to 20 wt %.

It is furthermore preferable for the first and/or second liquid film to contain hydrogen chloride (HCl) in a concentration of from 0.2 to 2.0 wt %. This addition promotes the removal of metallic contaminants such as ions of the metals copper, iron and nickel.

The first and second liquid films are preferably at room temperature (25° C.). The temperature may however be lower or higher than this, and it may be up to 95° C. The temperatures of the first and second liquid films may be the same or different.

The second liquid film is preferably removed by displacing it with a washing agent, for example ultrapure water, ultrapure water containing ozone, SC1 solution or dilute hydrochloric acid. The semiconductor wafer may subsequently be dried, for example by spinning the washing agent off from the semiconductor wafer at a high rotational speed with an influx of nitrogen, or by carrying out a drying method known as Marangoni drying.

EXAMPLE

Silicon semiconductor wafers with a diameter of 300 mm were cleaned after chemical-mechanical polishing (CMP). Coarse polishing agent residues were removed beforehand with the aid of rollers. Between 200 and 500 particles with an average size of more than 65 nm were detected on the polished surface of the semiconductor wafers pretreated in this way. Some of these semiconductor wafers were subjected according to the invention to two-stage cleaning with a cleaning liquid containing HF and O₃. During the first cleaning stage, the polished surface of the semiconductor wafers was sprayed in a single-wafer treatment device with an aqueous solution which contained hydrogen fluoride in a concentration of 0.1 wt %. At the same time, an oxygen/ozone mixture with 230 g of ozone/Nm³(stp) was passed through the gas space of the device. A liquid film containing ozone, with an HF concentration of 0.1 wt %, was formed on the surface of the semiconductor wafer treated in this way. After 30 s had elapsed, the semiconductor wafer was sprayed in a second cleaning stage with an aqueous solution which contained hydrogen fluoride in a concentration of 0.05 wt %. The ozone supply was not changed in any way so that a liquid film containing ozone, with an HF concentration of 0.05 wt %, was formed on the polished surface. After a further 30 s had elapsed, the cleaned semiconductor wafer was washed with ultrapure water and dried. The entire cleaning (first and second cleaning stages) lasted 60 s, and was carried out by rotating the semiconductor wafer with a constant speed of 300 rpm.

For comparison, semiconductor wafers which had been pretreated as indicated above were subjected to merely one-stage cleaning in the same device. The polished surface of the semiconductor wafer to be cleaned was sprayed with an aqueous solution which contained hydrogen fluoride in a concentration of 0.05 wt %. At the same time, an oxygen/ozone mixture with 230 g of ozone/Nm³(stp) was passed through the gas space of the device. A liquid film containing ozone, with an HF concentration of 0.05 wt %, was formed on the surface of the semiconductor wafer treated in this way. During the cleaning, the semiconductor wafer was rotated with a constant speed of 300 rpm. After 60 s had elapsed, the cleaned semiconductor wafer was washed with ultrapure water and dried. For some of the semiconductor wafers, the duration of the cleaning was extended to 240 s, and for others the concentration of hydrogen fluoride was increased to 0.1 wt %.

Examining the efficiency of the cleaning revealed that on average 80% fewer particles were found after the cleaning on the semiconductor wafers cleaned according to the invention. With the one-stage cleaning, 45% of the particles were removed in the best case. The effect of extending the cleaning time was that the number of detectable particles increased owing to etching effects, which reduced the efficiency of the cleaning.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. 

1. A process for cleaning a semiconductor wafer surface, comprising forming a first aqueous liquid film on the surface, the first liquid film containing hydrogen fluoride and ozone; replacing the first liquid film with a second aqueous liquid film which contains hydrogen fluoride and ozone, the concentration of hydrogen fluoride in the second liquid film being lower than in the first liquid film; and removing the second liquid film.
 2. The process of claim 1, wherein no more than 120 s elapse from formation of the first liquid film until removal of the second liquid film.
 3. The process of claim 1, wherein no more than 60 s elapse from formation of the first liquid film until replacement of the first liquid film with the second liquid film.
 4. The process of claim 1, wherein the concentration of hydrogen fluoride in the first liquid film is from 0.1 to 10 wt %.
 5. The process of claim 2, wherein the concentration of hydrogen fluoride in the first liquid film is from 0.1 to 10 wt %.
 6. The process of claim 1, wherein the concentration of hydrogen fluoride in the second liquid film is from 0.001 to 0.1 wt %.
 7. The process of claim 2, wherein the concentration of hydrogen fluoride in the second liquid film is from 0.001 to 0.1 wt %.
 8. The process of claim 4, wherein the concentration of hydrogen fluoride in the second liquid film is from 0.001 to 0.1 wt %.
 9. The process of claim 1, wherein the first liquid film is replaced with the second liquid film through displacement of the first liquid film by the second liquid film, and the surface remains constantly wetted with liquid during the replacement.
 10. The process of claim 1, wherein the semiconductor wafer is rotated with a rotational speed in the range of from 100 to 2000 rpm during cleaning.
 11. The process of claim 1, wherein the first or second liquid film, or both liquid films, contain HCl in a concentration of from 0.2 to 2.0 wt %.
 12. The process of claim 2, wherein the first or second liquid film, or both liquid films, contain HCl in a concentration of from 0.2 to 2.0 wt %.
 13. The process of claim 4, wherein the first or second liquid film, or both liquid films, contain HCl in a concentration of from 0.2 to 2.0 wt %.
 14. The process of claim 6, wherein the first or second liquid film, or both liquid films, contain HCl in a concentration of from 0.2 to 2.0 wt %.
 15. The process of claim 1, wherein the second liquid film is displaced by ultrapure water, ultrapure water containing ozone, SC1 solution or dilute hydrochloric acid. 